Logging Summary

IODP Expedition 308:

Gulf of Mexico Hydrogeology

Introduction

Figure 1. Bathymetric image of the continental slope of the Gulf of Mexico. Indicated are the drilling locations at the Brazos-Trinity Basin #4 (Site U1319, U1320, and U1321) and the Mars-Ursa Basin (Site U1322, U1223, and U1324).

Integrated Ocean Drilling Program (IODP) Expedition 308 was the first part of a two-component program dedicated to studying overpressure and fluid flow on the Gulf of Mexico continental slope (Figure 1), where rapid sedimentation overlaying a mobile salt substrate is the driving force behind many of the active processes present. Drilling and logging operations were performed at two basins in the northern Gulf with very different sedimentation histories.

The operations were aimed at evaluating the impact of different depositional settings and rates on sediment properties and fluid flow. The Brazos-Trinity Basin #4 (Figure 1), one of a chain of five local basins, is considered a classic model for the formation of deep-water turbidite deposits. In contrast to this is the Ursa Basin, 300 km to the NE of Brazos-Trinity Basin #4, where high sedimentation rates created overpressure. The sedimentary succession in the Ursa Basin is composed predominately of non-permeable mudstone above permeable interbedded sand and mud representing the “Blue Unit.” Consequently, drilling operations in the Ursa Basin were challenging due to the risk of creating shallow water flows by penetrating overpressured units. The measuring-while-drilling and logging-while-drilling operations were crucial to the primary goal of Expedition 308 and provided the best means to drill and provide physical measurements in this overpressured basin.

Logging Tools

Figure 2a. Schematic showing the configuration of the wireline logging tool strings that were used during IODP Expedition 308.

Figure 2b. Results of triple combo measurements and sonic tool string measurements from Hole 1301B. Black dots in the porosity, density, and P-wave velocity panels represent shipboard measurements on core samples.

Figure 2c. Masuring while drilling and logging while drilling (MWD/LWD) tool string used at each site.

Table 1. Summary of holes logged and tools deployed during IODP Expedition 308. See text for details.

Table 2 . Measurements recorded during measuring while drilling (MWD) and logging while drilling (LWD) operations

The logging program on Expedition 308 wasdesigned to obtain data needed to illuminate controls on slope stability, seafloor seeps, and large-scale fluid flow. Standard wireline tools -- the triple combo, the FMS/Sonic and the Well Seismic Tool (WST) -- were deployed at Hole U1320A (Figure 2a, Table 1).

The second wireline operation at Hole U1324A was run with a modified tool string and the WST (Figure 2b) to supplement the measuring-while-drilling (MWD) and logging-while-drilling (LWD) operations. Details on standard wireline tools used during Expedition 308 can be found here.

Drilling in the overpressured Ursa Basin carried potential risks and the MWD/LWD operation was therefore crucial to the primary goal of Expedition 308. These also provided the best means to drill and provide physical measurements in this overpressured basin. The LWD and MWD tools used during Expedition 308 included the GeoVISION Resistivity (GVR) tool, the Array Resistivity Compensated (ARC) tool, the Power Pulse measuring-while-drilling (MWD) tool, and the VISION Density Neutron (VDN) tool. Figure 2c shows the configuration of the MWD/LWD bottom hole assembly (BHA), and Table 2 lists the set of measurements recorded. This was the first time the ARC tool was used during an ODP or IODP expedition.

The GVR tool provided shallow-, medium- and deep-focused resistivity measurements of the formation and azimuthally orientated images of the borehole wall. The ARC tool is capable of multi-depth borehole compensated real-time and memory resistivity and gamma radiation measurements. The measured resistivity utilizes electromagnetic wave propagation in the formation as opposed to current flow to the formation in the GVR tool. Also included in the collar of the ARC is the Annular Pressure-While-Drilling (APWD) sensor. The APWD sensor is capable of measuring the borehole annulus pressure and temperature. The MWD PowerPulse tool transmits data by generating a continuous mud-wave transmission within the drilling fluid and by changing the phase of this signal (frequency modulation) to transmit relevant bit words representing information from various sensors. The VDN tool provides a borehole-compensated density measurement, a photoelectric effect value, and a neutron porosity measurement.

Logging Operations and Technical highlights

Following completion of coring in Hole U1320A, the logging operations in the Gulf of Mexico began with three wireline deployments. In the first pass with the triple combo, an obstruction at 173 mbsf prevented the tool string from reaching the bottom of the hole. This obstruction was passed during the second triple combo run, which reached the total depth of 299.6 mbsf. The FMS/Sonic tool string deployment reached the total depth of the borehole with both main and repeat passes up to pipe depth at 62.5 mbsf. The lockable flapper valve (LFV) temporarily obstructed the tool string entry into the borehole. The LFV also obstructed lowering the WST tool into the borehole and seawater was pumped to help open the valve.

Hole U1320B was the first MWD/LWD hole drilled during Expedition 308, followed by U1319B and U1321A. The identical bottom hole assembly and tool configuration was used for each Brazos-Trinity site. The total depth at each hole was reached with penetration rates of ~25 m/hr. Real-time data were transmitted to the surface at a rate of 24 Hz. Hole U1320B was drilled 20 m deeper than the adjacent cored hole to ensure that the topmost sensors in the MWD BHA recorded measurements to the total depth of Hole U1320A.

Logging operations at Brazos-Trinity Basin #4

Logging operations at Ursa Basin

Drilling in the Ursa Basin carried the significant risk of encountering shallow-water flow. This potential risk occurs when overpressured and unconsolidated sands are penetrated and flow into the borehole. These sands can be expelled at the seafloor and may lead to slope instabilities. To counter this problem, the identical MWD/LWD bottom hole assembly and tool configuration assembled for the Brazos-Trinity Basin #4 was used to drill at the Ursa Basin.

The first MWD/LWD dedicated hole in the overpressured Ursa Basin was drilled at Site U1322. For the first time in the history of IODP (and DSDP/ODP), MWD/LWD was tested as a viable tool to monitor real time pressure in a hole before coring the site. MWD/LWD drilling proceeded without incident at Hole U1322A and U1324A. At Hole U1323A, a ~3-m thick sand layer was encountered at approximately 198 mbsf and a pressure increase of 150 psi over the background drilling pressure in the APWD log was observed. The overpressure was stabilized and drilling operations continued after a wiper trip and the pumping of weighted mud in the hole. At 242 mbsf, a rapid drop in gamma radiation, suggestive of a second potentially overpressured sand interval, was observed in the data and drilling operations were terminated. A free-fall funnel was dropped on the seafloor to facilitate re-entering the hole, the MWD/LWD BHA was tripped to the surface, and cementing operations took place in order to comply with the Expedition 308 operations protocol. A subsequent camera survey showed no evidence of flow.

Drilling objectives for Site U1323 were achieved in three different ways: (1) overpressure was evidenced during LWD/MWD operations, (2) the IODP approach to “riserless-controlled drilling” proved efficient in controlling the flow, and (3) data obtained at Site U1323 provides information on the lateral continuity and the stratal architecture of the Ursa Basin.

Following completion of the MWD/LWD operation in Hole U1324A and prior to cementing, two wireline logging tools were deployed to obtain data that were not acquired with the MWD/LWD string. A tool string consisting of the HNGS, the DSI, and the GPIT was deployed first and the end of pipe was set at ~49 mbsf. An obstruction was encountered at 507 mbsf and logging commenced from this point. Although the same obstruction was encountered during the second pass, logging commenced this time from 509.5 mbsf. The second wireline deployment employed the WST to conduct a checkshot survey. The LFV obstructed lowering the WST into the open hole and seawater had to be pumped to help open the valve. A bad electrical line to the main trigger box and a corroded plug connecting the electrical leads to the GI gun also delayed the checkshot survey.

Logging Summaries

Logging operations during Expedition 308 at Brazos-Trinity Basin #4 and Ursa Basin provided a large volume of high quality data. This summary highlights some key points; for further discussion of the data and a detailed geological background the reader is referred to the Expedition 308 Preliminary Report and the Expedition 308 Proceedings Volume.

Brazos-Trinity Basin #4

Figure 3. BSummary of the wireline logging results obtained at Hole U1320A

Figure 4. FMS images of Hole U1320A showing evidence for potential slump surfaces, high angle deformation and lithological changes.

Figure 5. Summary of measuring while drilling and logging while drilling (MWD/LWD) results obtained at the Brazos-Trinity Basin #4 Hole U1319B.

Figure 6. Summary of measuring while drilling and logging while drilling (MWD/LWD) results obtained at the Brazos-Trinity Basin #4 Hole U1320B.

Figure 7. Summary of measuring while drilling and logging while drilling (MWD/LWD) results obtained at the Brazos-Trinity Basin #4 Hole U1321A.

Figure 8. Resistivity image of Hole U1320B showing east-west orientated breakouts.

Figure 9. GeoVISION resistivity (GVR) image of Hole U1320B. Apparent are thin sand beds within Lithostratigraphic Subunit IIe of Hole U1320B and a sharp contact with the top of Lithostratigraphic Unit III that correlates to seismic reflector R30.

Figure 10. GeoVISION resistivity (GVR) image of Hole U1320B revealing the existence of steeply dipping beds within a clay-rich unit corresponding to Lithostratigraphic Unit V.

Figure 3 summarizes the wireline logging results obtained at Hole U1320A. The data allow for a clear separation of the logged lithologies into two large intervals on the basis of changes in velocity, resistivity, bulk density, and porosity. These intervals generally correspond to the major lithology changes identified in the recovered cores. Low gamma radiation values (<60 gAPI) can be correlated to sand-prone layers in the core samples, whereas clay layers display higher values (>70 gAPI). However, low gamma radiation values observed in lithostratigraphic Unit III may be related to the abundance of calcareous foraminifera and microfossils usually characterized by low radiogenic element concentration. Low resistivity, gamma radiation, density and compressional velocity values correspond to borehole washouts. These result in prominent anomalies, in particular at depths of 80, 110, 140 and 170 mbsf. Below 176 mbsf the borehole is characterized by systematic density increase, interrupted only by a sand-rich layer at 230 mbsf, indicative of increasing compaction with depth.

FMS images of Hole U1320A (Figure 4) show evidence for potential slump surfaces, high angle deformation and lithological changes, which could contribute to seismic reflections. The resistivity contrast between the sand and clay sediments compares well with other log curves. However, at increased caliper values, the quality of the FMS images in these sediment sections is not reliable. Preliminary analyses of the FMS images show that in many intervals the borehole is irregular, resulting in an uneven contact of the FMS pads with the borehole wall. Nevertheless, good quality images in some intervals provide information that could not be gleaned from the cores, particularly where sedimentary and structural features were severely disturbed by the XCB coring process.

MWD/LWD operations at Holes U1319B, U1320B, and U1321A provide data coverage by all tools over the cored intervals (Figures 5, 6 and 7). In addition, the MWD/LWD data supplement the intervals not covered by the wireline measurements at Hole U1320A (Figure 3). Overall, hole quality in the uppermost borehole sections was in general more variable, with several caliper measurements reaching at least 43 cm. However, gamma radiation, resistivity and bulk density results obtained at Hole U1319B suggest a normal compaction trend in the clay-rich section of Site U1319 (Figure 5). In contrast, data from Hole U1320B and Hole U1321A are highly variable, with a series of intercalated sand and clay intervals dominating the uppermost stratigraphy. Low density and high porosity values also reflect the potential presence of sand units and corresponding enlarged borehole dimensions. The data trends in the lower part of these holes are signatures of a normal compaction trend where pore volume and water content decrease with depth because vertical effective stress is increasing.

Resistivity images of Hole U1320B and U1321A show apparent breakouts at the bottom of each borehole with an east-west orientation (Figure 8). These breakouts indicate a north-south maximum horizontal stress direction that can be attributed to lateral loading by high input of siliciclastic material derived from river plumes, turbidity currents, and/or slump deposits on the basin flanks. The resistivity images are also characterized by a series of thin alternating resistive and conductive laminations that may represent variations in silt content. Steep features at the bottom of Hole U1321A have been identified as potential slump deposits or faulted blocks.

The GVR resistivity images also proved useful in identifying thin sand beds within Lithostratigraphic Subunit IIe of Hole U1320B and a sharp contact with the top of Lithostratigraphic Unit III that correlates to seismic reflector R30 (Figure 9). In addition, the resistivity imaging at this hole reveals steeply dipping beds within a clay-rich unit corresponding to Lithostratigraphic Unit V (Figure 10).

The data acquired during the MWD/LWD operations at Brazos-Trinity Basin #4 make a bed-to-bed correlation between the sites possible. The acquired data are highly valuable for the study of sandy turbidites. Drilling at Site U1321 was the first MWD/LWD experience during Expedition 308 in the normally pressured Brazos-Trinity Basin #4 and an excellent exercise in preparation for MWD/LWD drilling in overpressured sediments at Ursa Basin.

Ursa Basin

Figure 11. Summary of measuring while drilling and logging while drilling (MWD/LWD) results obtained at Hole U1322A in the Mars-Ursa region. Resistivity and gamma radiation measurements show the highest variability and allow correlation between several units defined by visual observation of the cores and to seismic Reflectors S10 and S30.

Figure 12. Summary of measuring while drilling and logging while drilling (MWD/LWD) results obtained at Hole U1323A in the Mars-Ursa region (see Figure 1). The logging data correlate well to the data obtained at Site U1324 and Site U1322.

Figure 13. Summary of measuring while drilling and logging while drilling (MWD/LWD) results obtained at Hole U1324A in the Mars-Ursa region (see Figure 1).

Figure 14. GeoVISION electrical images obtained at Site U1322 reflecting the occurrence of undisturbed sediments but also of contorted and faulted sediments.

Figure 15. GeoVISION electrical images obtained at Site U1322. The most striking features are parallel east-west orientated contours of analogue resistivity that may represent breakouts indicating the direction of the minimal horizontal stress.

Figure 16. Resistivity image data showing several highly deformed intervals in Hole U1324A confirming the original logging-seismic interpretation and the presence of several mass transport deposits.

Hole U1322A represents the first drilling location in the Ursa Basin and is characterized by relatively uniform logging data, mostly indicating clay, mud, and occasionally silt (Figure 11). Resistivity and gamma radiation measurements show the highest variability and allow correlation between several units defined by visual observation of the cores and to seismic Reflectors S10 and S30. In general, gamma radiation and resistivity log responses proved to be particular useful in identifying lithostratigraphic units at each Ursa Basin site. The logging data strongly support the division of the lithostratigraphic column encountered into lithostratigraphic units (e.g. Units I and II) and subunits (Subunit Ia and Ib). These are well characterized in the LWD and wireline log responses (Figures 11, 12, and 13). Despite not coring Site U1323, the good-quality logging data (Figure 12) proved to be valuable for analysis of the stratigraphic history of the Ursa Basin. The logging data correlate well to the data obtained at Site U1324 and Site U1322 for the upper borehole section and confirm the predominance of mud and clay rich units, including two mass transport deposits.

The GVR electrical images obtained at Site U1322 and U1324 reflect the occurrence of undisturbed sediments but also of contorted and faulted sediments (e.g., Figure 14). The most striking features are parallel east-west orientated contours of analogue resistivity that may represent breakouts (Figure 15) indicating the direction of the minimal horizontal stress. Resistivity image data show several highly deformed intervals confirming the original logging-seismic interpretation of the presence of several mass transport deposits (e.g., Hole U1324A, Figure 16). These mass transport deposits also display characteristics of lower bulk density and resistivity compared with surrounding undeformed sediments. Site U1324 logging data also provide a detailed picture of the bedding style and lithofacies overlying the “Blue Unit” and allow for interpreting the evolution of the eastern levee of the Southwest Pass Canyon channel-levee system.

Conclusions

During Expedition 308, seven holes were logged, in generally excellent conditions, providing an exceptional data set to help characterize the spatial variation in composition, deformation, and rock properties in a flow-focusing environment. These data provide valuable insights into basin dynamics and basin fill in space and time. The resolution of the MWD/LWD and wireline logging data obtained are high enough to allow a bed-by-bed correlation of the basin fill.

The success of the logging program included the first-ever attempt in the Ursa Basin within IODP (and DSDP/ODP) to use MWD/LWD as a predictive tool for coring, and to make an assessment of flow of overpressured fluids into the drill hole in real time. This capability was put to the test when drilling in Hole U1323A encountered an overpressured sand unit and annular pressure recorded a sudden and substantial increase. It was demonstrated that situations like this can be controlled, and operations can be safely concluded under the operations protocol for Expedition 308.

Marc Reichow: Logging Trainee, Department of Geology, University of Leicester, Univresity Road, Leicester, LE1 7RH, UK

Gerardo J Iturrino: Logging Staff Scientist, Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades, NY 10964, USA